With the rapidly expansion of the market for laptop computers, mobile phones, electric vehicles, and the like, a secondary battery having a high energy density is required. Examples of a method for obtaining a secondary battery having a high energy density include a method in which a negative electrode material having a large capacity is used, and a method in which an nonaqueous electrolyte liquid having an excellent stability is used.
Patent Document 1 discloses using a silicon oxide or a silicate as a negative electrode active substance of a secondary battery. Patent Document 2 discloses a negative electrode for a secondary battery which has an active substance layer containing a carbon material particle that can absorb and desorb lithium ion, a metal particle that can be alloyed with lithium, and an oxide particle that can absorb and desorb lithium ion. Patent Document 3 discloses a negative electrode material for a secondary battery which is formed by coating the surface of a particle, which has a structure in which a silicon fine crystal is dispersed in a silicon compound, with carbon.
Also, the use of electrolyte liquid containing ionic liquid has been suggested in terms of imparting flame retardancy, as disclosed in Patent Documents 4 to 6.